1. Field of the Invention
This invention relates to a backup apparatus for ignition and fuel system which is designed to work upon occurrence of a failure in a main circuit of the system.
2. Description of Related Art
FIG. 1 is a block diagram showing the structure of a conventional backup apparatus for ignition and fuel system, wherein an electronic control unit is encircled by a chain line. The electronic control unit is connected with a pressure sensor 8 for detecting the pressure inside an inlet pipe of an engine (not shown), a water temperature sensor 9 for detecting the temperature of cooling water in the engine, a crank angle sensor 10 for detecting a predetermined crank angle of the engine, an ignitor 11 constituting a part of the ignition device, and an electromagnetic fuel injection valve 5 for feeding a jet of fuel to the engine when it is opened.
Moreover, the electronic control unit is comprised of a first input interface circuit 100 (hereinafter referred to as a first IF100), a second input interface circuit 101 (hereinafter referred to as a second IF101), a microcomputer 103, a failure judging circuit 104, an ignition system backup circuit 105, a fuel system backup circuit 106, a changeover circuit 107, an ignition system output circuit 108 and a fuel system output circuit 109. More specifically, the first IF100 shapes waveform of an angle pulse outputted from the crank angle sensor 10, while the second IF101 removes noise components from a pressure signal of the pressure sensor 8 and a temperature signal of the water temperature sensor 9 and shapes waveforms of both signals. The microcomputer 103 outputs an ignition signal for the ignition device and a drive signal for opening the fuel injection valve 5 upon receipt of signals from the first IF100 and the second IF101. The failure judging circuit 104 judges whether the microcomputer 3 is broken or not on the basis of a watchdog signal inputted from the microcomputer 3. The ignition system backup circuit 105 generates an ignition signal and outputs it upon receipt of the angle pulse from the first IF100. On the other hand, the fuel system backup circuit 106 receives the angle pulse from the first IF100 and a pressure signal and water temperature signal from the second IF101, thereby generating a drive signal for fuel injection and outputting it to the changeover circuit 107. The changeover circuit 107 selectively outputs signal received from the microcomputer 103 or those from the ignition system backup circuit 105 and the fuel system backup circuit 106 based on an output signal of the failure judging circuit 104. Then, the ignition system output circuit 108 current-amplifies the ignition signal selected by the changeover circuit 107 and outputs it to the ignitor 11. The fuel system output circuit 109 current-amplifies the drive signal selected by the changeover circuit 107 and outputs it to the fuel injection valve 5.
The operation of the conventional backup apparatus in the above-mentioned structure will be disclosed hereinbelow in a detailed manner.
The microcomputer 103 receives the angle pulse from the crank angle sensor 10 via the first IF100 thereby to obtain the rotating number of the engine. At the same time, the microcomputer 103 calculates the ignition timing from the obtained rotating number of engine, and the supply amount of fuel based on the rotating number of engine and the pressure signal from the pressure sensor 8 via the second IF101. Thereafter, the calculated amount of fuel supply is corrected by the microcomputer 103 based on the water temperature signal from the water temperature sensor 9 via the second IF101. Thus, the microcomputer 103 sequentially outputs to the changeover circuit 107 an ignition signal satisfying the ignition timing and a drive signal for fuel injection in compliance with the corrected amount of fuel supply in synchronous manner with the angle pulse.
The ignition system backup circuit 105 generates the ignition signal and outputs it to the changeover circuit 107 in synchronous manner with the angular pulse inputted from the crank angle sensor 10 via the first IF100. The fuel system backup circuit 106 generates the drive signal for fuel injection in correspondence to the pressure signal inputted from the pressure sensor 8 via the second IF101, and outputs it to the changeover circuit 107 in synchronous manner with the angle pulse inputted from the crank angle sensor 10 via the first IF100.
The failure judging circuit 104 which receives the watchdog signal from the microcomputer 103 determines that the microcomputer 103 is operating normally so long as receiving the watchdog signal in a predetermined cycle, and outputs an "L" level signal to the changeover circuit 107. On the contrary, when the watchdog signal is interrupted or not generated at all, the failure judging circuit 104 determines the microcomputer 103 operating in failure, and outputs a "H" level signal to the changeover circuit 107. The changeover circuit 107 selects and outputs the ignition signal and drive signal from the microcomputer 103 while it receives the "L" level signal from the failure judging circuit 104. However, the changeover circuit 107 selects and outputs the ignition signal from the ignition system backup circuit 105 and the drive signal from the fuel system backup circuit 106 while it receives the "H" level signal from the failure judging circuit 104.
The ignition system output circuit 108 current-amplifies the ignition signal selected by the changeover circuit 107 and supplies it to the ignitor 11 thereby bringing about ignition. The fuel system output circuit 109 current-amplifies the drive signal selected by the changeover circuit 107 and supplies it to the fuel injection valve 5, thereby opening the valve 5.
FIG. 2 shows a block diagram of another structure of the conventional apparatus.
In FIG. 2, the same and corresponding parts to those in FIG. 1 are designated by the same references and the description thereof will be abbreviated here. When the ignitor 11 starts operating upon receipt of the ignition signal, it generates an ignition diagnosis signal S.sub.1 indicating that ignition has been correctly achieved. The electronic control unit shown herein includes a third input interface circuit 102 (hereinafter referred to as a third IF102) which shapes waveform of the ignition diagnosis signal S.sub.1 and then outputs the signal S.sub.1 to the microcomputer 103. After having confirmed the ignition by the signal S.sub.1, the microcomputer 103 generates the drive signal for fuel injection. The drive signal is not outputted by the microcomputer 103 without the confirmation of the ignition. Since the apparatus operates in the same manner as the one in FIG. 1 in other points, description will be abbreviated.
In the foregoing structure of the conventional backup apparatus for ignition and fuel system, even if the ignition backup circuit 105 breaks down when the microcomputer 103 is not operating normally, the drive signal is eventually supplied through the fuel system backup circuit 106, the changeover circuit 107 to the fuel system output circuit 109, whereby the fuel injection valve 5 is opened. As a result, the following problems arise; the fuel is undesirably accumulated in the cylinder of the engine without ignition as it is fed successively, the catalyst for purifying the exhaust gas may generate heat and break, and so forth.